This invention relates to soldering and, more particularly, to the soldering of printed circuit boards by passing them through a standing wave of molten solder.
A known technique for processing printed circuit boards involves the tinning or coating of the printed circuits with solder, by passing the boards with their printed circuit surfaces in contact with a smooth standing wave of molten solder. The boards may also be fluxed, before passing through the standing wave of molten solder, by passing the boards through a standing wave of liquid flux. The standing wave of molten solder is produced by causing an upward flow of molten solder through a nozzle having a substantially rectangular discharge opening extending laterally of the wave, so that the molten solder overflows the edges of the nozzle and returns to a solder tank. The wave may be either one-sided, in which case it overflows only one longer edge of the nozzle, or it may be double-sided, in which case it overflows both longer edges of the nozzle discharge opening.
In recent years so called chip components have been developed by electrical component manufacturers, principally in Japan. These chip components may be manufactured in different ways but the finished product is essentially the same regardless of the manufacturing technique. Each chip component is very small, of the order of several mm, and has a body of molded plastics or ceramic material provided with two or, three in the case of transistors, conductive contact pads. The components are essentially leadless although the contact pads may project outwardly from the body. The chip components are generally rectangular or cylindrical and can be used to replace discrete resistors, capacitors, inductors, diodes and transistors.
The main attractions of the chip components are the dramatic increase in packaging density, reduction in production costs and improved product reliability. In use, the chip components are first bonded to the circuit side of a printed circuit board using an adhesive after which the contact pads of the chips are soldered to the conductive pads on the boards. Severe problems in achieving satisfactory soldering using conventional wave soldering techniques have been encountered. The problems manifested themselves in the occurence of random solder skips (non soldered joints) and bridging. The problems are believed to result from particular characteristics of the chip components and their mounting on the printed circuit boards. In the first place, because of the very high packaging density the spaces between the chip components are often very small and this coupled with the non-wettability of the component bodies tends to repel the solder from locations where it is desired. Furthermore, gases evolved from the adhesives and/or flux tends to cause solderless pockets. Finally, if a chip component happens to be orientated such that one of its contact pads is shielded from the direct flow of solder less solder than necessary will be deposited at that pad.